There is no reason to turn oneself inside out over it: shooting at and hitting a rapidly moving target (i.e. airplane) from a respectable distance is a non-trivial exercise and would take an atrocious amount of practice. In WWII and earlier multiple machine guns were used for the purpose, with the success rate (hits per rounds expended) being abominably low. And they had an advantage of using tracer rounds and correcting their aim in the process.
In World War II I believe the figure was 8,000 German 88 Anti-aircraft rounds fired(mind you, these are big shells that airburst so you don't need a direct hit) for each Allied Bomber shot down.
The threat from terrorists shooting 50 calibers at airborne aircraft is for all intents and purposes, non-existent.